Melting temperature and a precursor phase change in bacteriorhodopsin as function of pH

Abstract

In a recent review in this Journal, one of us has considered melting and precursor cooperative phenomena in chemically bonded assemblies. Here, we move from the phase diagram of simple molecular solids (e.g. nitrogen), as a function, say, of applied pressure, p, to that of bacteriorhodopsin (bR) which is one of the most frequently studied membrane proteins whose X-ray structures have been determined. However, instead of the physical variable, p, Heyes and El-Sayed have measured the melting and pre-melting transition temperatures as a function of pH. In the present study, we make proposals concerning the statistical-mechanical interpretation of their measurements in terms of additional degrees of freedom coming into play above the pre-melting temperature.

Keywords:

• Melting
• Precursor
• transitions
• Bacteriorhodopsin

Acknowledgements

One of us (NHM) wishes to acknowledge that his contribution to this study was made during a visit to DKFZ, Heidelberg. He is grateful to DKFZ for generous hospitality in the form of a scholarship.